Printing by active tiling

ABSTRACT

A photosensitive area  11 , such as a photolithographic sheet, in an images plane is notionally subdivided in both dimensions to form contiguous (tiled) sub-areas. Variable illumination means  1,4  provides a selected pixellated light pattern, which is directed  8, 9, 10  to fill a selected one of the sub-areas so that pixels of said pattern are, at least 15 microns across at the sub-area, and control means are responsive to an input signal representative of an image conjointly to control the production and direction of the pixellated patterns so that an entire image is produced over all of the said sub-areas. As shown, the variable illumination means comprises a light source  2  with digital micro-mirror array deflector device  4 , and the sub-area is selected by lens array  8  with a shutter  10  and polariser array  11 . The latter may be replaced by a two-axis steering mirror and lens array. An analogue micro-mirror array, optionally with, a kaleidoscope, may be used in the illumination means, with (a) collimating optics and lens array; or (b) a focussing macro-lens, for sub-area selection.

[0001] The present invention relates to the use of active tiling in twodimensions for exposing a photosensitive surface to an optical patternor image. It has particular but not exclusive relevance to thepreparation of printing plates, especially those of relatively largedimensions where it is desired to maintain a good resolution.

[0002] While it is possible to take a displayed image, e.g. from aliquid crystal matrix, and transfer it to a photoresist to create a maskor printing plate, there are size versus resolution limitations. Forexample, a plate of say 10 by 20 inches at a resolution of 600 dpi wouldrequire 6000 by 12000 pixels. It is currently not practical, and maybenot possible, to produce a passive matrix addressed liquid crystalspatial light modulator having these numbers of edge contacts.Furthermore, 600 dpi equates to a pixel size of 40 microns, and thisagain is not possible for current active matrix liquid crystal displays.

[0003] The invention uses an active tiling technique for the piecemealselective exposure of a photosensitive surface, whereby it is possibleto use a device with relatively low numbers of pixels to synthesise animage having a much larger number of pixels. In this way a large imagecan be recorded on the photosensitive surface with good resolution.

[0004] The use of active tiling in the preparation of holographic imagesis known. In such a system, sub-images are tiled onto the surface of anoptically addressed spatial light modulator (OASLM), e.g. a bistableelectro-optic liquid crystal device incorporating a photoconductivelayer, and the resulting image is illuminated with coherent light,normally in the visible range so that the resultant image can bedirectly viewed. An example is disclosed on UK Patent Application No2330471 (Secretary of State for Defence).

[0005] This type of holographic image preparation system only works whenthe pixels are suitably small, preferably of the order of the wavelengthof the coherent radiation, but in present practice somewhat larger ataround 6 microns, which size nevertheless remains very difficult toobtain. It should be noted that the above-mentioned patent applicationrefers to the use of demagnifying optics to decrease the effective pitchof each pixel in the spatial light modulator(s), i.e. as incident on theOASLM. In addition, in this type of application a very large number ofpixels is required (say around 10¹⁰) for a workable system, and in oneapproach a 3-inch diagonal OASLM supports 25 (5 by 5) tiled images(25×10⁶) pixels forming one channel, there being a collection of suchchannels which provide a section (only) of the complete hologram.

[0006] Technical requirements for a mask printing system are different.Shorter wavelength UV light is commonly used, with a different type ofphotosensitive material such as a photoresist/photocurable material, orconversely a, material which becomes removable, e.g. soluble, whenoptically exposed. The resolution necessary for printing is much lowerthan for holographic systems, and so less demanding. As noted above, 40microns equates approximately to 600 dpi, and would be a commonrequirement, although 20 microns or even 15 microns could be preferredin some applications.

[0007] Additionally it is known to use a liquid crystal matrix forselectively exposing photosensitive material in applications involvingthe printing of typed material see for example U.S. Pat. No. 4,830,468(Xerox) and U.S. Pat. No. 4,783,146 (Xerox) in which a linear array ofliquid crystal shutters is used to print an image one line at a time;and U.S. Pat. No. 3,824,604 (Stein) and U.S. Pat. No. 4,429,369 (Stanly)which employ a liquid crystal matrix to form alphanumeric characters fortransmission to a xerographic machine or photographic material. In thistype of apparatus, the array either extends over the whole of one of thedimensions of the photosensitive surface (or that area thereof to beused), or the size of the basic alphanumeric image is very small and ismoved relative to the receiving surface in some way. Text formed fromalphanumeric characters differs from other types of image in that aspace is naturally provided between each pair of characters so thattiling need not be as precise as in images where there is a degree ofcorrelation of image content between tiles.

[0008] In a first aspect, the invention provides apparatus for exposinga photosensitive area in a predetermined image plane which is notionallysubdivided in both dimensions to form contiguous sub-regions, theapparatus comprising variable illumination means for producing aselected pixellated light pattern, directing means for directing saidpattern to fill a selected one of said sub-regions, and control meansresponsive to an input signal representative of an image conjointly tocontrol said illumination means and said directing means such that theentire image is produced over all of the sub-regions in said plane. Forproduction of a printing plate, for example, the pixels need be nosmaller than 15 microns at the image plane, preferably no smaller than20 microns. While they can be significantly larger, e.g. 100 microns,according to desired result, it is preferred that they are no largerthan 40 microns.

[0009] The illumination means may be a light source (for example anarray of light emitting diodes, VCSELs—vertical cavity surface emittinglasers, or an electroluminescent array) or a light modulator receivingillumination from a separate source. Typical such, light modulators areliquid crystal arrays and micro-electromechanical (MEM) devices, such asan array of very small movable mirrors. The amount of contrast availablewill be determined at least in part by the type of illumination meanswhich is used.

[0010] A typical MEM mirror array comprises 1000 by 1000 pixels or 2000by 2000 pixels, each pixel being provided by a mirror switchable betweentwo states, e.g. plus and minus 10° relative to the plane of the array.These devices would require 12 by 12, or 6 by 6 replications(sub-regions) for a 20 by 20 inch photosensitive area. Also known areMEM mirror arrays in which the angular deflection of each mirror maytake a selected one of three or more values, e.g. seven as particularlydescribed later. In the simplest from of construction, deflection ofeach mirror element is about a single axis; but arrangements where eachelement is deflectable about two axes have been proposed.

[0011] In certain instances, for example where it is desired to expose aphotosensitive surface to ultra-violet (UV) or near UV light (blue endof the spectrum), the wavelength limitation may prevent the use ofcertain optical elements (for example liquid crystals tend to degradewith excessive UV illumination, or are incapable of providing therequired modulation), and may limit the choice of illumination means. Insuch a case, or where a broad optical range of illumination wavelengthmay need to be employed for different purposes, the use of an MEM devicemay be preferred by virtue of its relative insensitivity to wavelength.It is also relatively insensitive to high radiation intensities, anddoes not require polarising optics.

[0012] Preferably, the illumination means is electrically addressed,although other pixelwise addressing means may be used which are knownper se for the type of illumination means adopted.

[0013] Often the illumination means will not contain any facility fordirecting the light therefrom. In such a case, the directing means willbe entirely separate therefrom, and may comprise a variable lightdeflector or spatial selector for selecting the sub-region to beilluminated, and/or even a means for moving the photosensitive area. Inother cases, light direction provided by the illumination means is inone direction only (e.g. an array of micro-mirrors tiltable about asingle axis to a selected one of a plurality of angles, and a separatemeans maybe required for the second direction, e.g. a variable lightdeflector, spatial selector or moving means as before.

[0014] In one type of arrangement light from the illuminating means isspread over a wide angle, for example using a diffuser at an imageplane, and multiple images of the illuminating means are formed, forexample using a lenticular lens array. A suitable spatial selector forreceiving the resulting light comprises collimating optics for receivinglight from the illuminating means, followed by replication optics, suchas a lenticular lens array each lens of which receives the same patternof illumination from the collimating means, and a spatial shutter arrayfor selecting the lenticular lens corresponding to the selectedsub-region. The spatial shutter could be any known suitable device, butone device preferred for its speed comprises an array of nematic liquidcrystal π cells.

[0015] However, it is often simpler and more efficient to use a lightdeflector capable of acting in both dimensions. Any suitable lightdeflector or combination of light deflectors (one per dimension) knownin the art may be used for this purpose, but a preferred device isprovided by a two-axis steerable mirror, which again has the virtue ofbeing relatively insensitive to wavelength and high radiation intensity.In an embodiment, a light pattern from an illuminating means is directedvia focussing optics, such as a lens, onto a two-axis steerable mirrorfollowed by a focussing lenticular lens array. The steerable mirror iscontrolled to select which lenticular lens is illuminated, correspondingto the selected sub-region.

[0016] Either of the above arrangements can be used in conjunction withan MEM mirror array operated digitally, i.e. each single mirror of thearray providing a pixel of the pattern) is operated digitally either todirect light toward the predetermined image plane or in anotherdirection.

[0017] In other instances, the illuminating means may itself contributeto the determination of the light direction, and so form part of thelight directing means. A typical and preferred example thereof is an MEMmirror array operated in an analogue manner. For the provision of apattern for a selected sub-region, the array is operated in aquasi-digital manner as above. However, the all mirrors providing thelight to be directed towards the image plane are deflected by the sameangle, which can be varied according to the location of the selectedsub-region. Thus, such a device operated in this manner could befollowed by a kaleidoscopic system and a focussing maro-lens, the angleof deflection of the mirrors of the array determining which area of thelens is illuminated for transmission to an adjacent (selected)sub-region. In this arrangement the kaleidoscope provides evenillumination of the spatial light modulator and multiple images areavailable therefrom by internal reflection—which of the multiple imagesactually contains the light energy depends on the deflection angleprovided by the mirrors. Alternatively, light from an analogue operatedMEM mirror array could be passed by collimating optics such as amacro-lens, a selected region of which is illuminated as before, to aselected one of lenticular lens array corresponding to the selectedsub-region. Unlike the two arrangements discussed in the previous threeparagraphs, both of these arrangements require light which is relativelywell collimated.

[0018] In a second aspect, the invention provides a method of exposing aphotosensitive area, in response to an input signal representative of animage, said area being notionally subdivided in both dimensions to formcontiguous sub-regions, comprising the steps of poviding a light patterncorresponding to a selected part of said image and directing saidpattern to fill a corresponding selected one of said sub-regions andrepeating the process for other sub-regions until the entire image isproduced over all of the sub-regions of said area.

[0019] Further details and advantages of the invention will becomeevident on a reading of the appended claims, to which the reader isdirected, and upon a consideration of the following description ofembodiments of the invention, made with reference to the accompanyingdrawings, in which:

[0020]FIG. 1 shows a first embodiment of the invention using a digitallyoperated MEM mirror array in conjunction with a spatial shutter array;

[0021]FIG. 2 shows a second embodiment of the invention using adigitally operated MEM mirror array in conjunction with a two-axissteerable mirror;

[0022]FIG. 3 shows a third embodiment of the invention using an analogueoperated MEM mirror array in conjunction with collimating optics and alenticular lens array; and

[0023]FIG. 4 shows a fourth embodiment of the invention using ananalogue operated MEM mirror array in conjunction with a kaleidoscopeand focusing optics.

[0024] In the Figures the same reference number is used in respect ofcorresponding elements performing similar functions.

[0025] In FIG. 1 light from a lamp or laser source 1 is directed via alens 2 and a bean splitter 3 onto a micro-electromechanical array 4 of1000 by 1000 mirrors each of which can be deflected between two angles.Light 7 reflected from mirrors at one of the angles is lost from thesystem, but diverging light 5 reflected from mirrors at the other angleand transmitted through the splitter 3 is collimated by a lens 6 and isthen incident on a 12 by 12 lens array 8. Each lenticular lens forms aseparate image of the array 4.

[0026] One such image is selected by a corresponding 12 by 12 array ofshutters formed by a liquid crystal array 10 of π cells and a 12 by 12array of Glan-Taylor polarisers 11 (a single large polariser could beused). Lights incident on the liquid crystal array should be polarised,and this is effected either by a separate polariser, for exampleimmediately following the lamp 1, by using a laser source 1, or byproviding a polarising beam splitter 3. In use a selected π cell isactivated to rotate the plane of incident light by 90° to enabletransmission by the corresponding polariser 10 in known manner so thatan image of the whole array 4 is formed on a selected sub-area in animage plane 11.

[0027] In operation of the system, an input signal representative of animage which is very large in terms of numbers of pixels, for example12000 by 12000, is effectively broken down to sub-image signalsrepresentative of 12 by 12 contiguous (tiled) sub-images. Each sub-imagesignal in turn is used to address the mirror array 4, while informationconcerning the location of the sub-image within the entire image is usedto control the shutter array so that the sub-image is directed to thecorrect location in plane 11, so that eventually the entire image isreproduced in plane 11. The sub-images may be provided in anypredetermined order, but preferably immediately adjacent sub-images areformed in immediate succession, for example serially along a first lineof 12 sub-images, the serially along succeeding lines, either in thesame line direction (as in conventional raster scanning) or in reversedirections (as in boustrophedral scanning).

[0028] In use, a photosensitive surface is located in plane 11. Wherethis surface is for production of a printing mask or plate, e.g. aphotoresist layer, it may be necessary to use a UV source 1.

[0029] In this system it must be ensured that the light 5 issufficiently diverging for uniform illumination of the lens array 8.With certain types of light source or illumination optics, or where analternative type of light modulator replaces the mirror array 4, thismay occur naturally, but in other cases additional means known per seare provided to ensure that this happens.

[0030] Generally it is preferred to avoid the use of polarisationoptics, and FIGS. 2 to 4 have no such requirement. In these Figures thelight source may provide polarised (for example laser) or unpolarisedlight.

[0031] In FIG. 2, the diverging light beam 5, is focused by a lens 12via a mirror 13 and a 12 by 12 lens array 14 onto the imaging plane 11.The mirror 13 is tiltable about two axes so that a selected one of thelenses of array 14 receives substantially all of the sub-image light 5from the array 4 and produces a corresponding sub-image at its locationin plane 11, so that again a complete image may eventually besynthesised thereon by tiling. In this arrangement, the light fromsource 1 need not be collimated. As shown and preferred, the mirror isplanar, but a mirror curved in one or both dimensions could be employedto act in conjunction with lens 13, or even alone (although thisrequires a larger movable component) for focussing purposes.

[0032]FIG. 3 is somewhat similar to FIG. 1, but here the mirror array 4is replaced by a 2000 by 2000 micro-electromechanical array 15 ofmirrors which can be tilted in at least 7 different directions, onedirection producing light 7 lost to the system, leaving at least 6useful directions. In this case the light incident on the mirror arrayis arranged to be reasonably well collimated, so that light reflectedfrom mirrors of the array set to one of the useful angles is transmittedby the splitter 3 to fall on a corresponding restricted area of acollimating lens 6 and thence to a corresponding one of a 6 by 6 array16 of lenses illuminating a corresponding local area of plane 11.

[0033] In use a sub-image signal is fed to array 15 and its location onthe plane 11 is selected in at least a first dimension by selection ofthe deflection angle of the selected mirrors of array 15. For anysub-image, this deflection angle is selected from the six useful angles,and is common to all selected mirrors for that sub-image. If the mirrorsof the array 15 are capable of multi-angle deflection about two axes(six useful direction for each axis), this alone enables selection ofthe sub-image location in both dimensions, and again the complete imageis synthesised by tiling of the sub-images on plane 11.

[0034] Alternatively some other manner of providing selecting of thesub-area location in the second dimension must be provided—for example,the position of the photosensitive surface in plane 11 could be changed,the entire mirror array 15 could be rotated about the second axis, or afurther steerable deflecting mirror could be located between splitter 3and lens 6. It will be noted that as illustrated FIG. 3 does away withthe need for the shutter array of FIG. 1.

[0035] The arrangement of FIG. 4 avoids the requirement for the lensarray 16 of FIG. 3 by the use of a kaleidoscope 17 between the splitter3 and the mirror array 15. Sub-image light emerging from splitter 3after reflection at array 15 falls onto a local area of a lens 18 independence On the deflection angle of the micro-mirrors, and the lens 18focuses it onto a corresponding local area of plane 11 so that tiling ofsub-images in at least one direction may be accomplished. Arrangementsfor tiling in the second dimension are much the same as those discussedwith reference to FIG. 3.

[0036] It should be clear to the skilled reader that these embodimentsare exemplary only, and that various modification may be made within thescope of the invention defined by the appended claims. For example,although FIGS. 1 and 2 have been described with respect to atwo-dimensional placement of each sub-image in plane 11 by control ofthe shutter array 10, 11 or the two-axis mirror 13, it should be clearthat placement in one dimension may be so provided (i.e. a linearshutter array, or a one axis mirror) together with an alternative methodof controlling placement in the other dimension. As discussed withrespect to FIGS. 3 and 4, this could be provided, for example, bymovement of the photosensitive surface, provision of a further one-axistiltable mirror (so that in FIG. 2 there will be two such mirrors inseries), or by tilting of the array 4.

1. Apparatus for exposing a photosensitive area in an image plane whicharea is notionally subdivided in both dimensions to form contiguoussub-areas, the apparatus comprising variable illumination means forproducing a selected pixellated light pattern; directing means fordirecting said pattern to fill a selected one of said sub-areas so thatpixels of said pattern are at least 15 microns across at said sub-area,and control means responsive to an input signal representative of animage conjointly to control said illumination means and said directingmeans such that the entire image is produced over all of the saidsub-areas.
 2. Apparatus according to claim 1 wherein said pixels are atleast 40 microns across at said sub-area.
 3. Apparatus according toclaim 1 or claim 2 wherein said illumination means provides ultra-violetlight.
 4. Apparatus according to any preceding claim wherein theillumination means is provided by a pixellated light source. 5.Apparatus according to any preceding claim wherein the illuminationmeans comprises a light source and a pixellated spatial light modulator.6. Apparatus according to claim 5 wherein the spatial light modulator isa micro-electromechanical device.
 7. Apparatus according to claim 6wherein the micro-electromechanical device comprises a two-dimensionalarray of tiltable mirrors.
 8. Apparatus according to any preceding claimwherein the illumination means is optically addressable to produce saidlight pattern.
 9. Apparatus according to any one of claims 1 to 7wherein the illumination means is electrically addressable to producesaid light pattern.
 10. Apparatus according to any preceding claimwherein the illumination means and the directing means are separateelements.
 11. Apparatus according to any preceding claim wherein thedirecting means comprises means for replicating said light pattern toform an array of like patters, and, means for selecting one of saidpatterns for transmission to a corresponding sub-area of said area. 12.Apparatus according to claim 11 wherein said replicating means comprisesa lens array.
 13. Apparatus according to any one of claims 1 to 10wherein the directing means comprises means for focusing said lightpattern and a mirror tiltable about at least one axis for receiving saidfocussed light pattern for directing the pattern to a selected saidsub-area.
 14. Apparatus according to claim 13 wherein said mirror istiltable about two axes.
 15. Apparatus according to claim 13 whereinsaid directing means includes means for moving said photosensitive areain said image plane.
 16. Apparatus according to any one of claims 13 to15 wherein a lens array is located between the tiltable mirror and theimage plane.
 17. Apparatus according to any one of claims 1 to 9 whereinthe control means and the illumination means form at least part of thedirecting means, being arranged so that light from the pixels of thepattern is directed towards the image plane at any selected one of aplurality of angles, said selected angle determining at least in partthe selected sub-area.
 18. Apparatus according to claim 17 and claim 7wherein each of the array of mirrors is tiltable about two axes tothereby determine the location of the sub-area in two directions. 19.Apparatus according to claim 17 and claim 7 wherein each of the array ofmirrors is tiltable about one axis to determine the location of thesub-area in one direction, and light from the array of mirrors isdirected to the image plane via a mirror tiltable about a different axisto determine the location of the sub-area in a second direction. 20.Apparatus according to claim 17 and claim 7 wherein each of the array ofmirrors is tiltable about one axis to determine the location of thesub-area in one direction, and means are provided for moving thephotosensitive area in the image plane to determine the location of thesub-area in a second direction.
 21. Apparatus according to any one ofclaims 18 to 20 wherein light from said array of mirrors is directed tosaid image plane via collimating optics and an array of lenses eachcorresponding a said sub-area.
 22. Apparatus according to any one ofclaims 18 to 20 wherein light from said array of mirrors is directed tosaid image plane via a kaleidoscope and a focusing lens.
 23. A method ofexposing a photosensitive area, in response to an input signalrepresentative of an image, said area being notionally subdivided inboth dimensions to form contiguous sub-regions, comprising the steps ofproviding a pixellated light pattern corresponding to a selected part ofsaid image and directing said pattern to fill a corresponding selectedone of said sub-regions and repeating the process for other sub-regionsuntil the entire image is produced over all of the sub-regions of saidarea, wherein pixels of said light pattern are at least 15 micronsacross at said photosensitive area.
 24. A method according to claim 24wherein said photosensitive area comprises a recording medium.
 25. Amethod according to claim 24 wherein said recording material isselectively cured, or selective rendered removable, on exposure to lightfrom said illumination means.
 26. A method according to any one ofclaims 23 to 25 wherein said light from the illumination means comprisesultra-violet light.
 27. A method of producing a printing plate includingthe step of performing the method of any one of claims 23 to
 26. 28.Apparatus for exposing a photosensitive area substantially ashereinbefore described with reference to any one of FIGS. 1 to
 4. 29. Amethod of exposing a photosensitive area substantially as hereinbeforedescribed with reference to any one of FIGS. 1 to 4.